[Update, 5:29 p.m. May 20: Oklahoma authorities said at least 37 people were killed in the tornado, and that the number would rise. Hospitals were treating scores of casualties, including children.]

Two elementary schools were destroyed, and an untold number of homes and businesses sustained heavy damage near the cities of Moore, Newcastle and Oklahoma City. CNN reported that rescue crews swarmed over Plaza Towers Elementary School, where 75 students and staff had sought refuge in a hallway.

Britane Diacon-Boese of Oklahoma City was worried about students she works with. "I have clients who can't be found," she said.

The other school hit by the tornado was Briarwood Elementary School in Oklahoma City. Local newscasters reported that children were trapped inside.

Even for Oklahoma, famous for tornadoes, this was a significant storm. Some people likened it to a massive cluster of tornadoes that hit on May 3, 1999, one of the largest and deadliest storms in state history.

"Our house is gone," one woman in Moore told a reporter with KWTV-9, crying and clutching her two children's hands. "Everything but where we were is gone."

The woman said she and her children hid in their bathtub with a mattress over them. Had they been anywhere else in the house, they would have been killed, she said.

In southwest Oklahoma City, cars were smashed and sitting upside down atop houses that had been reduced to rubble. People dug through destroyed homes even as strong storms remained a threat.

The National Weather Service reported that destructive hail the size of tennis balls is expected throughout the afternoon. Severe storms are expected to continue through the afternoon and evening.

Giant hail was falling as Chad Bartlett and his wife, Helen, were driving to his son's high school in Oklahoma City on Monday afternoon. The drive was treacherous.

"With the rain you couldn't see the car in front of you," Bartlett said.

The roads were congested as other people tried to flee the area. "There was wind and hail. The radios were blaring with the National Weather Service and sirens," said Bartlett, a pastor. "I drove to the school just praying the whole way."

In a phone interview, Bartlett said they arrived to see teachers taking students down the basement for shelter. He and his wife picked up their son and drove off. They drove through the town of Moore, just ahead of the tornado. They kept driving and Bartlett told his wife and son, "Just watch for the funnel cloud. Just watch."